Pressure slip casting involves the use of porous plastic molds to fabricate green ceramic components. It has advantages over conventional non-pressurized slip casting processes in that the cycle time is reduced, as well as part-to-part variation, and material properties are improved. Pressure slip casting is currently used in industry for producing green ceramic parts from high clay content compositions with relatively coarse particle size distributions. For example, see the article entitled "Bodies for Fast-Cycle Sanitaryware Production" by M. Vouillemet et al, published in Interceram, Vol. 39, No. 1, 1990, pp. 17-23. Commercially available pressure slip casting machines and the technology to fabricate porous plastic molds are available from various machine manufacturers. An example of one such manufacturer is NETZSCH Inc.
In the pressure slip casting process, the mold employed has both passages for draining the water, which is forced from the molding surface of the mold out of the mold cavity via a porous mold layer to the outside of the mold during the high pressure phase of the slip casting cycle, as well as passages for supplying compressed air and water into the porous layer to spurt water or air through the porous layer and out into the molding cavity from the molding surface when the cast product is to be removed from the mold. If the mold is constructed of an upper part or top part and a lower or bottom part, for example, the product can not be removed simultaneously from the upper and lower parts. In one current technique of demolding, therefore, one mold part is evacuated by applying vacuum to attract the product, whereas the other mold part is supplied with compressed air to remove the product. Then, the vacuum is released to supply compressed air to that one part thereby to remove the product. The mold passages are also used to evacuate the porous layer during the demolding step, and for mold flushing and cleaning. Further background information relative to pressure slip casting mold construction and methods of making of molds may be found in U.S. Pat. Nos. 4,884,959; 4,913,640; 4,913,868; 5,087,399; as well as certain of the references cited therein.
In slip casting, a ceramic powder which may consist of a wide variety of clay, silicas and silicon compounds, as well as various metal oxides, is mixed with appropriate additives to promote densification and to impart the desired material properties in the final structure. This powder is mixed with a liquid vehicle, typically water, as well as with dispersants and organic binder in such a way that the mixture attains a suitable low viscosity for pouring or pumping into a porous mold. The porous mold absorbs excess liquid vehicle, leaving a solid compound of ceramic powder and binder, saturated with liquid vehicle in the spaces between ceramic particles. The mold thus acts as a filter, the water being the filtrate, whereas the solid particles in suspension and separated out in this filtering action are left behind as a build-up layer on the wall of the mold cavity to form a hollow article (or a solid article, if desired) having an exterior surface complemental to the shape of the mold cavity surface. The pressure slip casting is then removed from the mold and dried to remove the residual liquid vehicle. The organic binder may be removed by a thermal process involving liquification, pyrolysis and distillation. The resulting porous ceramic green body may be densified and strengthened by hot isostatic pressing or kiln sintering.
Some relatively recent research and development efforts directed to pressure slip casting processes have been focusing on the problems associated with applying this process in porous plastic molds for producing hollow-shaped green ceramic components from slip compositions with little or no clay content and submicron particle size distributions. Since the filtering channels in the porous plastic mold material are usually no smaller than 50 to 150 microns in cross-section in order to maintain adequate mold porosity to achieve practical production cycle times, the challenge in casting these submicron particle size compositions has been to adjust the casting slip properties and molding process parameters so that a firm layer of particulate material will build-up on the porous plastic mold surface when hydrostatic pressure is applied to the casting slip. If such submicron particles are fully dispersed in the liquid vehicle, such solid particles will pass through the channels in the mold while entrained in the liquid vehicle without any of the desired build-up occurring on the mold cavity surface. In attempts to overcome this problem, a casting slip composition was modified to include a suitable flocculator to cause the slip composition solid particles to aggregate into small, loosely aggregated masses of material suspended in the liquid vehicle. If the casting slip is so flocculated, the floccules are too large in size to pass through the mold porosity channels and the desired build-up of solids on the mold cavity surface can be achieved.
However, it was then found that the degree of flocculation required to achieve successful slip casting with slip compositions having such submicron particle size distributions does not allow enough water removal from the casting layer flocculus during the conventional in situ water removal process step, at least within the cycle time constraints required to achieve economical production. Hence, at the end of this step, an excessive amount of water is retained in this cast layer. This in turn results in an as-cast part that lacks sufficient strength to be self-supporting when removed from the mold. Accordingly, the part will slump and deform from its intended shape due to its own weight before enough water can be removed in post-molding drying operations.